Method And Apparatus For Controlling The Recovery Of Solid Polyolefin From A Continuous Reaction Zone

ABSTRACT

A Polymerisation process and a loop reactor comprising polymerising olefins in a liquid diluent to produce a liquid slurry containing particles of normally solid polymer within the loop reactor, allowing the polymer to settle in a settling leg, periodically opening a 180° rotating product take-off valve located at the end of the settling leg to allow a charge of particles to flow out of the settling leg, the product take-off valve being operated by a pneumatically driven double-acting actuator, the pneumatic system being regulated by a system comprising pneumatic control valves, the improvement which consists in using automatic control valves, which are automatic v-ball valves.

This invention relates to the withdrawal of solid polyolefin from a slurry of such solids. In a particular aspect, it relates to a method and apparatus for controlling the recovery of particulate polyolefin from a slurry thereof, for example from a stream of polymerisation mixture continuously flowing in a loop reactor.

U.S. Pat. No. 3,242,150 disclosed an improvement to loop reactors consisting in adding to the bottom part of a loop reactor a receiving zone, since known as settling leg, wherein the solids settle by gravitation, and withdrawing a fraction concentrated in solids from said receiving zone.

U.S. Pat. No. 3,293,000 disclosed a loop reactor with several settling legs. Control of the valve is described at column 3, lines 2 to 22.

U.S. Pat. No. 3,374,211 disclosed a modified process for removing polymer.

More recently, U.S. Pat. No. 5,183,866 related to the employment of a flash line heater in conjunction with the periodic operation of a settling leg of a loop reactor. The process is characterised by the fact that the elongated zone is constructed such that the flow time of the charge of slurry in an elongated confined zone including the flash line heater is equal to at least about 25% of the time between the closing of the settling leg valve and the next opening of the settling leg valve.

U.S. Pat. No. 5,455,314 discloses a method for controlling in a continuous manner the withdrawal of a reaction slurry containing a polymer product from a polymerization reactor by manipulating continuously a V-notch ball valve in a primary effluent line and by automatically open a control valve in a redundant line as a backup for the primary control valve in the event the primary line becomes plugged.

The invention relates to a polymerisation process comprising polymerising olefins in a liquid diluent to produce a liquid slurry containing particles of normally solid polymer within a loop reactor, allowing the polymer to settle in a settling leg, periodically opening a 180° rotating product take-off valve located at the end of the settling leg to allow a charge of particles to flow out of the settling leg, the product take-off valve being operated by a pneumatically driven double-acting actuator, the pneumatic system being regulated by a system comprising pneumatic control valves characterised in that the control valves are V-ball valves.

The invention relates also to the use of V-ball control valves to regulate the pneumatically driven double-acting actuator operating a 180° rotating product take-off valve of the settling leg of a loop reactor.

Finally, the invention relates to a loop reactor comprising a settling leg, a 180° rotating product take-off valve located at the end of the settling leg, the 180° rotating product take-off valve being operated by a pneumatically driven double-acting actuator, the pneumatic system being regulated by a system comprising pneumatic control valves, characterised in that the control valves are V-ball valves.

Preferably, the control valves are automatic control valves.

The invention will now be described with reference to the drawings:

FIG. 1 is a schematic diagram of a loop reactor with two settling legs and their control system.

FIG. 2 is a schematic diagram of the control system.

FIG. 3 is a schematic diagram of the bottom part of a settling leg, showing the product take-off valve and its actuating mechanism.

FIG. 4 is a schematic diagram of the pneumatic system.

FIG. 5 is a schematic diagram of the electronic control system.

In the embodiment illustrated in FIG. 1, polymerisation is carried out in a loop reactor 10. The monomer and the diluent are introduced respectively through lines 14 and 16 merging into line 13, and the catalyst is introduced through line 17. A propeller 11 linked to a motor M circulates the mixture. As polymer particles are produced, they accumulate in settling legs 22. The settling legs are each provided with a product take-off valve (PTO valve or PTO) 23 connected to a conduit 20.

Referring now to FIG. 2, there is shown a loop reactor 10 provided with two settling legs 22 a and 22 b, each provided with a PTO valve (respectively 23 a and 23 b) controlled by a control unit 28.

FIG. 3 shows the bottom of a settling leg 22, with a PTO valve 23 connecting it to conduit 20. The PTO valve is a rotating valve, the rotation being controlled by a mechanism M.

The PTO valve 23 of the settling leg 22 is only periodically opened, whereby the polymer particles present in the settling leg 22 can pass into conduit 20. The opening time of the PTO valve 23 should be closely controlled, in order that substantially all particles present in the settling leg 22 pass into conduit 20, whilst substantially no monomer and diluent leave the reactor 10.

Two types of PTO valves are in use. The most common relies on a 180° rotation of the moving part of the valve, whereby the valve turns from closed (0°) to open (90°) then closed (180°); during the next cycle, the valve rotates backwards. Valves with a 90° rotation are also in use, whereby the moving part turns from closed (0°) to open (90°) then backwards to closed (0°). The present invention provides an improved control system for the first type of PTO valves.

PTO valves are generally pneumatically actuated. FIG. 4 a shows that each PTO valve 23 is provided with a double acting pneumatic actuator 40, which controls the speed at which it turns. In the case of a 180° rotation, the speed at which the PTO turns is particularly important in the sense that it directly controls the time it remains open.

The air flow sent to the double acting pneumatic actuator 40 is directed by a two-way system 45 driven by a solenoid. FIG. 4 b shows one position of the system 45, wherein air coming from conduit 50 is sent via conduit 42 into the actuator 40, returns via conduit 41 and exits through conduit 51. FIG. 4 c shows the other position of the system 45, wherein air coming from conduit 50 is sent via conduit 41 into the actuator 40, returns via conduit 42 and exits through conduit 52.

Pneumatically actuated PTO valves were always controlled by manually adjusting the outlet flow of air using control valves 61 and 62. There is provided a separate control for each valve 61 and 62, in case the ball in the PTO valve 23 would turn at different speed in each direction.

It has now been surprisingly found that control of a PTO valve 23 is improved by using automatic control valves 61 and 62. In a preferred embodiment, V-ball control valves are used. Such valves throttle using the rotation of a notched ball segment whose shape is such that it allows at the same time to have a very precise control of the flow air for small openings while having a full bore opening when needed. As an example of V-ball valve, there can be cited a Worcester V-flow control valve type V44-66UMPTN90.

It was not obvious to find appropriate control valves 61 and 62, because all control valves tested up to now did not provide a precise control of the flow of air, taking into account that the amount of air in the actuator 40 is relatively small. More importantly, it was not obvious that operation of the reactor 10 would be more stable.

The use of automatic control valves provides several advantages with regard to a better reliability of the PTO valves. Indeed, the frequent operation of the PTO valves, such as every 15 to 90 seconds, leads to the wear of said valves which then operate slower. This effect is immediately compensated by an automatic increasing of the amount of air needed by the actuators. An automatic control of the amount of air needed by the actuators avoids that the PTO valves get stuck in an open position. The use of automatic control valves allows also a more precise control of the amount of air entering into the actuators than the manual control valves do. When using manual control valves, there is always a risk that when reducing the amount of air entering into the actuators, the PTO valves finally get blocked in an open position leading to the depressurisation of the reactor. This may occur with the first reactor of a double loop system when long opening times of the PTO valve of said reactor are needed.

The use of automatic control valves and of 180° rotating PTO valves allows a good control opening time of said PTO valves. This could not be achieved in the same way by using 90° rotating PTO valves.

It has also been found that the inner volume of conduits 51 and 52 had to be reduced to the maximum possible without creating a restriction to airflow. Conduits 51 and 52 have a diameter ranging from 1.27 cm (½ inch) to 2.54 cm (1 inch), preferably said conduits have a diameter of about 1.9 cm (¾ inch). Conduits 51 and 52 have a length of less than 150 cm, preferably less than 100 cm. In a most preferred embodiment, conduits 51 and 52 have a diameter of about 1.9 cm (¾ inch) and a length of about 20 cm between the system 45 to the automatic control valves 61 and 62.

Referring now to FIG. 5, there is shown a preferred embodiment of the control mechanism. The PTO valve 23 is provided with sensors 71 and 72, located in the double actuating actuator, which indicate the position of the valve 23. Information from the sensors 71 and 72 is sent respectively via transmitter 73 and cables 74 and 75 to a computer 76 to determine the rotation time of the PTO valve. The rotation time of the PTO valve is sent to a rotation controller 79, which also receives a set point for rotation time 81 of the PTO valve from the operator. Depending on the difference between the rotation time of the working PTO valve and of the desired rotation time introduced by the operator, a signal 82 is sent to the control V-ball valves, the opening of the V-ball valve either increases by 1% at every cycle when the rotation time is slower than the operator set point or decreases by 1% when the rotation time is faster than the operator set time.

The set point of the rotation time of the PTO valve may be adjusted manually by the operator or controlled by the system as a function of the reactor pressure drop at each opening of the reactor.

EXAMPLE AND COMPARATIVE EXAMPLE

A loop reactor was fitted with a system according to the invention. The loop reactor had the following characteristics:

nominal capacity: 5.5 tons/hr.

volume: 19 m³

number of settling legs: 4

size of the settling legs: 20.3 cm

size of the flash lines: 7.6 cm

size of the PTO valves: 5 cm

The double acting actuators 40 of the PTO valves 23 were each provided with a control system according to FIG. 4. The conduits 41 and 42 had a 1.27 cm (½ inch) diameter and a 3 m length. The automatic control valves 61 and 62 were Worcester V-flow control valves type V44-66UMPTN90 with a Cv of 8, connected to the system 45 by conduits of 1.9 cm (¾ inch) diameter and 20 cm length.

The stability of the operation of the reactor was measured in regard of the variations of the pressure measured in the reactor. The reactor operation was very stable. Indeed, said variations were lower by 25% when compared to those of a comparative reactor equipped with control valves 61 and 62 of the manually controlled type, the other characteristics of the comparative reactor being the same as those of the example. In addition, in the comparative reactor, the capacity of each settling leg was lower by 10%. 

1-6. (canceled)
 7. A method for operating an olefin polymerization loop reactor system comprising: a) introducing an olefin, a polymerization catalyst, and a diluent carrier liquid into a loop reactor, having an internal circulation pump; b) operating said circulating pump to circulate said diluent liquid and said olefin through said loop reactor while polymerizing said olefin monomer in the presence of said catalyst system to produce a slurry of polymer particles in said carrier liquid; c) diverting the flow of said slurry through said loop reactor into a settling leg connected to the loop reactor and having a rotating take-off valve which is operable to rotate from an initial 0° reference closed position to an intermediate open position and a final closed position in order to sequentially discharge settled polymer slurry from said settling leg to withdraw said polymer slurry from said reactor system; d) operating said rotating valve through a cycle of operation to rotate said valve from the initial closed position at a 0° reference to the intermediate position to open said valve and discharge polymer particles from said settling leg followed by continuing the rotation of said valve to the final closed position to close said valve followed by another cycle of operation in which the direction of rotation of said valve is reversed to rotate said valve from said final closed position to said intermediate open position to said initial closed position; e) determining the time of rotation of said valve from the initial 0° reference closed position to the final reference closed position; f) comparing said valve rotation time with a set point representative of a desired rotation time; and g) adjusting the speed of rotation of said take-off valve in response to the comparison of said valve rotation time with said set point rotation time in order to increase the speed of rotation when the valve rotation time is less than the set point rotation time and decrease the speed of rotation when said valve rotation time is faster than said set point rotation time.
 8. The method of claim 7 wherein said rotating take-off valve is a 180° rotating take-off valve which is operable to rotate from the initial 0° reference closed position to a 90° intermediate open position and a 180° final closed position and said valve cycle of operation to rotate said valve from the closed position at 0° reference to the 90° reference position to open said valve and discharge polymer particles from said settling leg followed by continuing the rotation of said valve to the 180° final position to close said valve is followed by the next cycle of operation in which the direction of said valve is reversed to rotate said valve from 180° to 90° in which said valve is open, to 0°.
 9. An olefin polymerization loop reactor system comprising: a) a loop reactor; b) at least one inlet for introducing an olefin monomer and a diluent carrier liquid into said loop reactor; c) a catalyst inlet for supplying a polymerization catalyst system to said loop reactor; d) a pump in said loop reactor effective for circulating said diluent liquid and olefin monomer through said loop reactor to provide for the polymerization of said olefin monomer in the presence of said catalyst system to produce a slurry of polymer fluff particles in said diluent carrier liquid; e) at least one settling leg connected to the loop reactor for receiving slurry from said reactor and sequentially discharging the settled polymer slurry from said at least one settling leg to withdraw polymer slurry from said reactor system; f) a rotating take-off valve in said settling leg which is operable to rotate from an initial 0° reference closed position to an intermediate open position and a final closed position in order to sequentially discharge the settled polymer slurry from said settling leg to withdraw said polymer slurry from said reactor system; g) a double acting activator for said take-off valve to rotate said valve from the initial closed position at a 0° reference to the intermediate position to open said valve and discharge polymer particles from said settling leg followed by continuing the rotation of said valve to the final reference position to close said valve followed by another cycle of operation in which the direction of rotation of said valve is reversed to rotate said valve from the final position to the intermediate position at which said valve is open to the initial reference position 0° at which said valve is closed; h) a pneumatic controller for said activator which functions to alternately direct pneumatic fluid to one side of said activator while opening the other side of said activator to exhaust; i) a supply and exhaust system for said pneumatic operator comprising at least one inlet conduit connected to a source of pneumatic fluid and another conduit connected to an exhaust zone for pneumatic fluid; and j) at least two of said conduits having automatic control valves for opening and closing said conduits.
 10. The system of claim 8 wherein said automatic control valves are pneumatic v-ball valves.
 11. The reactor system of claim 9 wherein said valve is a 180° rotating valve which is operable to rotate from the 0° reference closed position to a 90° intermediate open position and a 180° final closed position and said double acting activator rotates said valve from the closed position at a 0° reference to the intermediate 90° open position followed by continuing the rotation of said valve to the 180° position to close said valve through another cycle of operation in which the direction of rotation of said valve is reversed to rotate said valve from 180° to 90° at which said valve is open to 0° at which said valve is closed.
 12. The reactor system of claim 9 further comprising sensors associated with said valve to said valve being at the 0 reference closed position and the final closed position for generating signals representative of said valve reaching said initial closed position and said final closed position and a rotation controller representative of information generated from said sensor signals to control said pneumatic controller. 